A photoelastic modulator (PEM) is an instrument that is used for modulating the polarization of a beam of light. A PEM employs the photoelastic effect as a principle of operation. The term "photoelastic effect" means that an optical element that is mechanically stressed and strained (deformed) exhibits birefringence that is proportional to the amount of deformation induced into the element. Birefringence means that the refractive index of the element is different for different components of a beam of polarized light.
A PEM includes an optical element, such as fused silica, that has attached to it a transducer for vibrating the optical element at a fixed frequency within, for example, the low-frequency, ultrasound range of about 20 kHz to 100 kHz. The mass of the element is compressed and extended as a result of the vibration. The combination of the optical element and the attached transducer may be referred to as an optical assembly.
The compression and extension of the optical element imparts oscillating birefringence characteristics into the optical element. The frequency of this oscillating birefringence is determined by the length of the optical element and the speed of the transducer-generated longitudinal vibration or sound wave through the material that comprises the optical element.
The effect of the oscillating birefringence of the PEM on a linear-polarized monochromatic light wave is to vary over time the phase difference between the orthogonal components of the light that propagates through the optical element. This phase difference is known as retardation or retardance and can be measured in terms of length, waves (for example, quarter-wave, half-wave), or phase angle.
The accurate measure and control of retardation (by precise detection of the polarization of the PEM output light) has numerous practical applications. Certain applications may demand polarization measurement sensitivity levels on the order of 10.sup.-6.
The optical assembly is contained within a housing or enclosure that normally includes an aperture through which the light under study is directed through the optical element. The enclosure supports the optical assembly in a manner that permits the optical element to be driven (vibrated) within it to achieve the above noted photoelastic effect.
The optical assembly must be mounted to the enclosure in a way such that the mechanisms for mounting the optical assembly permit free vibration of the optical assembly without introducing any stress or strain on the optical element. Such stress or strain would result in undesirable changes in the birefringent characteristics of the optical element.
In the past, the optical assembly has been mounted within the enclosure with the use of elastomeric grommets or grommet-like members. The grommets were synthetic rubber, buna, or a silicon elastomer. The grommets were mounted to the enclosure on opposing sides of the optical assembly. Acrylic, cone-shaped supports were bonded to the optical assembly, and the grommets were located so that a cone-shaped support would protrude into the bore of the elastomeric grommet. Typically, the optical element was held between two opposing pairs of grommets. In other approaches, the grommets were supported on movable brackets. Once the optical assembly was in place (with the supports fit into the bore of the grommets), the brackets that hold the grommets were fastened to the enclosure.
The prior technique for mounting the optical assembly to the enclosure is generally effective but has at least two drawbacks.
One drawback is that the elastomeric grommets may not be used when the PEM is to be employed in a high or ultra high vacuum chamber as is sometimes required for certain applications. In such applications, the grommets tend to suffer from "outgassing" within the vacuum chamber, with the result that the desired vacuum level in the chamber may never be properly obtained, or undesirable contaminants are released from the grommets and interfere with expensive instrumentation and optics within the chamber. The grommets of the prior art could be formed of material that can withstand high temperature and high vacuum environments, but such material is costly.
Another drawback to the use of elastomeric grommets for supporting a vibrating optical assembly is the deleterious effects it has on the overall performance quality factor, or "Q" value, of the photoelastic modulator. In this regard, "Q" is defined as the ratio of the energy stored in a system to the energy lost per cycle. The higher the "Q," the more efficient the system. The elastomeric grommets tend to dampen the vibration of the optical element, thus requiring more drive energy to maintain the desired vibrational frequency of the element. Increasing drive energy increases the heat generated within the photoelastic modulator, which causes a reduction in the Q value.
The present invention is directed to an apparatus and method for mounting an optical assembly of a photoelastic modulator to a rigid enclosure. The technique is such that the problems associated with outgassing in a high vacuum environment are completely eliminated. Moreover, the "Q" value of the system employing the mounting techniques of the present invention will be greater than that of a system employing the elastomeric grommets described above.
In a preferred embodiment, the brackets and elastomeric grommets are eliminated in favor of an array of coiled springs that are attached between the optical element and the enclosure to suspend the optical element within the enclosure to facilitate its vibration. The springs are not subject to the outgassing problem of elastomeric members. The mounting technique is particularly suitable for modulators employed in high and ultra high vacuum environments. The springs are configured and arranged so that no stress or strain is induced into the optical element as a result of mounting the element with the springs.
In another embodiment of the present invention, the optical assembly is suspended between threaded members that are mounted to the enclosure and are extendable into contact with the optical assembly to support the optical assembly without deforming the optical element. In one version of this alternative embodiment, the tips of the threaded members engage posts that are mounted to the optical element. In another version, the tips of the threaded members engage correspondingly shaped recesses formed in the optical assembly.
There are other advantages of the present invention, which will become apparent upon reading the detailed description set forth below.